In a typical start system four a gas turbine engine, for example, one used in an aircraft auxiliary power unit, a low voltage brush-type series-wound DC motor is coupled to the gas turbine engine via a clutch and gearbox. The DC motor is electrically connected to a source of power, such as a battery or a transformer rectifier unit (TRU), via high current feeders.
To start the gas turbine engine, low-voltage, high current electrical power is supplied to the DC motor from the source of power and the clutch is engaged. Due to the inherent characteristics of the series wound DC motor, the motor draws maximum current and produces maximum torque when startup is commenced, and as startup progresses, the current drawn and the torque produced decrease from their initial maximum values. As a result, the gas turbine engine is initially accelerated very rapidly in an uncontrolled manner.
The above manner of starting a gas turbine engine has a number of problems. The high current initially drawn by the DC motor requires the use of relatively large high-current feeders and also causes the power source voltage to decrease or droop by an undesirably large amount during the time period immediately following the commencement of engine start.
Also, the rapid, uncontrolled acceleration of the engine due to the high initial torque generated by the DC motor is undesirable since the engine may be driven through the speed range within which the engine ignition may occur too quickly, resulting in failure to ignite the engine.
Various alternative starting systems have been devised. For example, instead of having a DC motor coupled to the gas turbine engine via a clutch, a synchronous machine operable in both a starting mode and a generating mode has been drivably coupled to a prime mover. In the starting mode, the synchronous machine acts as a motor to provide motive power to the engine. In the generating mode, the synchronous machine acts as a generator to convert motive power generated by the prime mover to electrical power. Examples of such alternative systems are illustrated in U.S. Pat. Nos. 5,013,929 to Dhyanchand, 5,055,764 to Rozman, et al. and 5,068,590 to Glennon, et al.